A mobile communication system employing a W-CDMA (Wideband Code Division Multiple Access) method adopts a frequency re-use scheme using the same frequency band among cells in order to improve frequency-utilization efficiency. This type of frequency allocation has great interference with neighboring cells, and a communication environment is largely different depending on user's location within the cell. Such being the case, this type of mobile communication system has a case in which transmit power control (TPC (Transmit Power Control)) is applied.
The transmit power control involves controlling reception power of the signal from each mobile terminal in the base station so that the reception power gets fixed by transmitting and receiving control information (TPC control bits) corresponding to an SIR (Signal to Interference Ratio) value calculated based on a pilot symbol between the base station and the mobile terminal.
The transmit power control is classified into inner loop control and outer loop control. The inner loop control is such that a TPC control bit value is obtained by comparing an SIR target value with an SIR measurement value calculated from the pilot symbol transmitted from a transmitting station, and the transmitting station controls the power of the transmission signal, corresponding to the TPC control bit value. The outer loop control is such that the SIR target value is updated with a proper value at a predetermined cycle through communications between an RNC (Radio Network Controller) and the base station.
On the other hand, in the mobile communication system using the W-CDMA method, an examination of adopting an adaptive modulation and coding (AMC (Adaptive Modulation and Coding)) method of dynamically selecting a communication-enabled coding and modulation method corresponding to a communication environment for actualizing faster communications, is underway. In the mobile communication system using the AMC method, the AMC corresponding to the communication environment of a location where the mobile terminal is present is executed even in such a case that the mobile terminal is present in whichever location within the cell. Accordingly, the control information necessary for executing the AMC needs surely transmitting to a cell edge. Further, a modulation and coding scheme (MCS: Modulation and Coding Scheme) set of the AMC needs preparing so as to be communication-enabled also at the cell edge.
FIG. 24 is a diagram illustrating a communication sequence for executing the AMC between a mobile terminal 2110 and a base station 2111. In FIG. 24, to begin with, the mobile terminal 2110 transmits a pilot signal to the base station 2111 (S2101). The base station 2111 estimates a channel status (e.g., an SINR (Signal-to-Interference and Noise power Ratio) value) based on the pilot signal. The base station 2111 transmits the SINR to the mobile terminal 2110 (S2102). At this time, the base station 2111 transmits scheduling information together for the mobile terminal 2110, which is determined based on the SINR.
The mobile terminal 2110 does not transmit a user data packet till its transmission is permitted by the scheduling information. The mobile terminal 2110, when the transmission is permitted, determines a modulation format (a coding and modulation method) corresponding to the communication environment of the self-terminal in a way that takes account of the SINR value, amount of a packet information etc contained in a transmission queue. The mobile terminal 2110 notifies the base station 2111 of the modulation format (S2103), and transmits the packet signal to which the thus-determined modulation format is applied (S2104). The base station 2111 sends “ACK” or “NACK” as a response to the mobile terminal 2110, corresponding to a reception status of the packet signal to which the modulation format is applied (S2105). Note that FIG. 24 illustrates a communication sequence related to an uplink to the base station from the mobile terminal, however, the same communication sequence related to a downlink to the mobile terminal from the base station is also executed.
By the way, in the communication sequence shown in FIG. 24, the information transmitted in S2102 and S2103 becomes control information for the AMC. Accordingly, in the case of employing the AMC method, this type of control information for the AMC needs transmitting and receiving via both of the uplink and the downlink.
Moreover, it is required that the control information for the AMC is surely transmitted up to the cell edge, and hence a gain of the control information signal is increased by applying “Repetition” and “Spreading” to the control information data. FIG. 25 is a diagram illustrating a communication method for the control information for the AMC and illustrates an example in which Repetition=16 is applied for assuring characteristics at the cell edge, and an 80-bit area is ensured as a communication field for the control information. According to this example, the control bits are allocated in [CB] areas of the communication field of the control information, and repetition data of the control information bits set in the [CB] areas are allocated in [CB(R)] areas adjacent to the [CB] areas on the right sides. Namely, according to the example in FIG. 25, 5-bit control information can be transmitted by using 80-bit control information communication field. Thus, there is a possibility of ensuring the control information communication field at substantially the same rate as that of the communication are ensured for the data communications.
For others, there are the following documents as documents that disclose the prior arts related to the invention of the present application.    [Patent document 1] Japanese Patent Laid-Open Publication No. 2004-72157    [Patent document 2] Japanese Patent Laid-Open Publication No. 2003-37554